. This application, written in response to PAR-18-039, will advance outcome measurement for sleep problems in children (age 3-12 years) with Autism Spectrum Disorder (ASD). Sleep problems (e.g., sleep disturbances and sleep-related impairments) in children with ASD range from mild to severe and affect as many as 80% in this age group (Katz et al, 2018).There is no accepted outcome measure for sleep problems in children with ASD. The absence of a relevant, reliable and valid outcome measure is a barrier to rigorous testing of behavioral or pharmacological treatments targeting sleep in this population. Using a mixed method design, this multisite, transdisciplinary project will build a new parent rating for sleep disturbances and related impairments in children with ASD. The study is guided by methods described in Patient Reported Outcome Measures (www.Fda.Gov/Cder/Guidance, 2009). This monograph compels investigators to include the patient population in measure development. Given the age range of this study and likelihood of cognitive or language delays in children with ASD, we rely on parents as primary informants. We have an outstanding team of investigators and consultants with expertise in measure development and sleep problems in ASD. In Year 1, up to six focus groups with parents will explore the onset and behavioral manifestations of sleep problems in children with ASD. The focus group material will be reviewed and coded to generate a bank of items (Bearss et al. 2016). These items will be reviewed for clarity and reading level by parents and an expert panel. In Year 2, the bank of items (version 1 of ASD-Sleep Questionnaire; ASDSQ) will be placed on a secure website to collect parent-reported data on sleep disturbances and related impairments in 1200 children with ASD. Items will be scored 0-3 (none, mild, moderate or severe). Factor analysis and item response theory will be applied to establish the factor structure and prune the item pool toward the final version (Scahill et al. in press). In Year 3 and early Year 4, an in-person clinical assessment of 135 children with ASD (age 3-12 years) will be conducted to collect reliability and validity data on the final ASDSQ version. A subsample of children will be invited to return for test-retest on ASDSQ, 2 weeks and 4 weeks later. In Year 4, data from the clinical assessment will be used to evaluate convergent validity, divergent validity and test-retest reliability of the ASDSQ. Also in Year 3 and early Year 4, we will collect five consecutive nights of actigraphy data in a total of 75 children with ASD (age 3 to 12) to evaluate sleep efficiency (percentage of time sleeping divided by total time in bed with lights off). Actigraphy-measured sleep efficiency (ASE) will be used to conduct exploratory analyses comparing ASDSQ scores in children with impaired ASE values (poor sleepers) to those with acceptable ASE values (good sleepers) (Goldman et al. 2009; Moore et al. 2017). Receiver operating characteristic curves will be used to explore the sensitivity and specificity of SDSQ on ASE classifications.